The present invention relates to rail switches, and more particularly to optimizing the geometry of rail switches with regard to high speed rolling stock, switch wear and maintenance, and passenger comfort.
For more than 150 years the method of switching railroad stock from one track to a departing track has been through the use of split switch turnouts. Split switch turnouts are composed of a pair of switch points to direct the rolling stock away from its original course, curved closure rails to direct them to the new course and frogs which allow outside wheels to pass over or through crossing rails. The switch points were fabricated from straight pieces of standard rail and are now called straight split switches. These straight split switches were economical and performed well enough on lightly used, slow speed tracks and still constitute the majority of switches in use today.
Approximately 40 years ago a need was perceived for a switch which would furnish higher safe speed and comfort. The result was the adoption of curved point switches. The radii of these curved points were generally greater than the closure curves and acted as transition or easement curves. They also reduced the switch point entry angle by about half. This has improved things greatly and these curved point split switches, in recent years, have generally been the choice in heavy rail and transit rail design when maximum speed and comfort was sought. This type of turnout has been used in the construction of recent light rail transit systems.
Still, the curved switch points were not tangent to the stock rail and the wheel flange encountering the switch point and being abruptly diverted laterally to a new direction produced two bothersome problems. The major one for heavy freight haulers was the heavy wear experienced by the switch points and the resulting high maintenance costs, as well as limitations of speed and scheduling. For transit operators the limitations on speed and scheduling are also important, but even more important was the sudden and in some cases severe lateral acceleration experienced by passengers.
Rail designers in the United States turned to European rail technology to solve these two problems, spurred by two developments in the United States. One was the decision taken by AMTRAK to initiate much higher speed train service, and the other was a move toward alternatives to heavy rail transit systems such as BART and MARTA. Europe and Japan have developed very high speed trains, such as the Bullet Train and the TGV, and European light rail systems are look to as a source of new rail technology. Indeed, many concepts of European rail practice have been or are being accepted in the United States.
The "new" technologies are not really new. They have been in use in Europe for some time, and they are not really new to the United States, either. For example, swing nose or movable point frogs have been used in the U.S.A., mostly in transit systems, and tangential geometry has been used in crane rail and transit systems. From the perspective of a light rail designer, many of the characteristics of European turnouts are expensive and add very little or nothing to the quality of the ride or rated speed of the AREA No. 20 turnouts now in use.
The one characteristic recognized as an improvement over an AREA standard turnout is the use of tangential geometry. However, claims of significantly higher speeds are not supported by experience. The basis of these claims is the reduction of the switch's point angle from 0.degree.27' to 0.degree. on a No. 20 turnout. This factor significantly improves the ride through the switch and in most regards the tangential geometry switch is superior to the AREA switch. However, when considering the physical relationships of the wheel flange encountering the switch point, it is apparent that a 0.degree. switch point angle is not necessary nor is it desirable.
The wheel flange rides from 3/8 inch to 3/4 inch inside the gauge line and does not encounter the switch point until it is well past the point of the switch. At this point, which we term the effective point of entry, a tangential geometry switch has developed an effective entry angle of 0.degree.21' to 0.degree.27'. In designing a rail switch for maximum comfort and speed it is the effective entry angle that should be minimized and the switch curve radius maximized.
In recognition of this fact the secant geometry switch has been propose. It is similar to the AREA geometry except that the radius of the point and the closure curve are the same. To compare the two geometries, the following table indicates the parameters of a standard AREA No. 20 switch, compared to the best tangential geometry switch and secant geometry switch that could be produced within the same lead as the No. 20 switch.
______________________________________ AREA Tangential Secant ______________________________________ Point Angle 0.degree. 27' 19" 0.degree. 0' 0" 0.degree. 04' 37" Entry Angle 0.degree. 32' 17" 0.degree. 20' 05" 0.degree. 20' 05" Closure Radius 3333.36 2444.22 2581.37 Point length 39' 0" 50' 65/8" 48' 6" Lead 156' 01/2" 156' 01/2" 156' O1/2" ______________________________________
While the tangential geometry switch has a lower entry angle than the AREA switch, it has three major drawbacks. The point is longer, the closure radius is much sharper, and the point is knife-edge and therefore weak and vulnerable. The secant switch, given the same lead length, is superior to the tangential geometry switch. Although the entry angle is the same, the point is shorter, the closure radius is larger (though still far smaller than the AREA geometry), and the point is less vulnerable.